The unique characteristics of ferroelectric materials have been known for many years. The development of ferroelectric components, especially capacitors, has experienced a renewed interest because its ability to store in a nonvolatile manner polarization states corresponding to binary values. With this capability, ferroelectric capacitors have been developed for use in digital integrated circuits. Indeed, the hysteresis characteristic of a ferroelectric capacitor lends itself well as the storage mechanism in nonvolatile memory cells.
In order to fully utilize the properties of ferroelectric components, it is necessary to determine the polarization state previously stored in the component. When the ferroelectric material is utilized as a dielectric in fabricating a capacitor, the effective capacitance is different, depending on whether the capacitor is polarized in one direction or the other. With this inherent characteristic, a ferroelectric capacitor can be discharged so that a voltage of one magnitude is developed if the ferroelectric capacitor were polarized in one direction, and a voltage of a different magnitude is developed if the ferroelectric capacitor were polarized in the opposite direction. The voltage developed as a result of the discharge of the ferroelectric capacitor can then be compared with a reference to determine the polarization state of the capacitor. When employed in connection with single-transistor, single-capacitor ferroelectric memory cells, such type of polarization state determination is known as single-ended sensing.
The traditional problem with single-ended sensing of the polarization state of ferroelectric capacitors is that the electrical charge discharged or read from a ferroelectric capacitor incurs short-term and long-term changes. Short-term changes in the characteristics of a ferroelectric capacitor occur when a cell is written a first time, and then immediately thereafter before the polarization state has settled back to a quiescent magnitude. As a result, an electrical charge discharged from the capacitor during both write operations is different, even when the polarization state remains the same. Long-term changes in the ferroelectric capacitor characteristics occur due to aging and fatigue phenomena. According to the aging characteristic, the polarization state magnitude of a ferroelectric capacitor gradually degrades over time. The magnitude also degrades by the fatigue phenomena as the number of polarization state changes increase as a result of writing the memory cell with different data.
In order to circumvent these problems with single-ended sensing of ferroelectric memory cells, complementary-type cell architectures have been developed which include a pair of ferroelectric capacitors and select transistors, where each ferroelectric capacitor is polarized to opposite states. A pair of bit lines is also employed with this type of cell architecture. When such a memory cell is read, the electrical charge discharged from the respective ferroelectric capacitors generates a different voltage on each bit line. The inputs of a differential sense amplifier can be connected to the pair of bit lines and differentially sense whether one bit line voltage is greater than the other, or vice versa, and thereby determine the polarization state of the pair of ferroelectric capacitors. With this scheme, each memory cell includes twice the number of components of a single-transistor, single-capacitor cell, and thus the density of the memory cells per chip is compromised.
From the foregoing, it can be seen that a need exists for a high density memory architecture utilizing single-transistor, single-capacitor ferroelectric cells and single-ended sensing, but where the reference voltage for the sensing circuits changes dynamically with the characteristics of the ferroelectric capacitors. An associated need exists for an improved reference circuit which generates a voltage corresponding to the instantaneous ferroelectric capacitor characteristics. Yet another associated need exists for a reference voltage generator that generates a voltage that tracks short-term and long-term changes in ferroelectric capacitor characteristics.